mongo2.dc1.ru.itoolabs.net has no CNAME records.
Question and response
Question
dig mongo2.dc1.ru.itoolabs.net. NS
Answer
The response has no answer records.
Authority
dc1.ru.itoolabs.net. 3429 SOA dns1.itoolabs.net. noc.itoolabs.net. 2026012001 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0